HONOLULU -- The Defense Department gave the Navy permission Tuesday to keep training with sonar for another two years, a move denounced by activists who say the sound waves can harm dolphins and other marine mammals.

Navy officials had sought the two-year exemption from the Marine Mammal Protection Act, allowed under the 2004 National Defense Authorization Act, saying they needed time to study how sonar use at major underwater training ranges affects the environment.

The environmental impact statements required by the Marine Mammal Protect Act will take about two years, Navy officials said.

The ranges are off Hawaii, Southern California and the East Coast.

"We cannot stop training for the next two years," said Don Schregardus, deputy assistant secretary of the Navy for the environment. "That would put our sailors in the Navy at considerable risk."

Sailors use active sonar by pumping sound waves through the ocean and listening to the echo as it bounces off underwater objects. Navy leaders have made practicing sonar techniques a top priority as other nations have bought more advanced diesel submarines, which are quieter than earlier models and thus harder to find.

Environmentalists cite incidents of whales, porpoises and dolphins that have become stranded en masse on beaches after being exposed to sonar.

"The Navy has more than enough room in the oceans to train effectively without injuring or killing endangered whales and other marine species," said Joel Reynolds, senior attorney at the Natural Resources Defense Council, which is suing the Navy over its sonar use. "Because the Navy trains with this dangerous technology in some of the richest underwater habitat on earth, it is legally obligated to take simple, commonsense steps to protect marine life."

Navy officials said they're claiming a two-year exemption because a federal judge in California ruled last year that the Navy needed to do more detailed analysis of the effect its sonar training would have on the environment.

"The courts have ruled we probably will need to do the full analysis," Schregardus said. "Our first goal is to comply with the law and get the permits we need under the Marine Mammal Protection Act as we found. ... This is pretty much our only recourse."

The Navy says the new exemption will allow sailors to go ahead with 40 separate exercises over the next two years.